This invention relates to improved fuel cell assemblies, and, in particular, to bipolar and unipolar separators for fuel cell assemblies with solid polymer electrolyte positioned between electrodes of the cell.
Fuel cells, which produce electricity from fuel containing hydrogen, methanol or ethanol and air or oxygen, have long been considered a viable alternative to power generation by combustion. Potential advantages of fuel cells include very low or zero emission, low noise and high efficiency. One of the most promising fuel cell technologies uses proton conductive membranes as electrolyte. It has been shown that such fuel cells have a great potential to supply electrical power for on-site generation in buildings, at remote locations, in portable power generators and in vehicles.
A typical fuel cell assembly, which uses proton conductive membranes, consists of stacked cell units. Each of the fuel cell units includes a membrane electrode assembly, which consists of a proton conductive membrane stacked between two electrodes. One electrode is the cathode (air and/or oxygen side) and one the anode (fuel side). At the cathode, the oxygen containing fluid reacts with the catalyst forming anions. The fuel reacts at the catalyst layer to form solvated hydrogen cations, which migrate through the membrane to the cathode. The cations react on the cathode with the anions to produce water, heat and electricity.
Membrane material can be a perfluorosulfonic ion exchange polymer sheet as sold by DuPont under its Nafion trade designation. The electrodes consist typically of porous carbon materials with catalyst layers containing platinum or platinum group metals as active components. Suitable membrane electrode assemblies can be obtained from several companies, such as W.L. Gore and Associates, Inc. of Elekton, Md., which sell them under the trade name PRIMEA Membrane Electrode Assemblies or from MER Corporation of Tucson, Ariz.
The membrane electrode assemblies are typically interposed between separator plates, which distribute fuel and oxidant to the respective sides of the membrane electrode assemblies, provide means for removing reaction products, serve as current collectors and structurally support the membrane electrode assemblies. The separator plates should be durable under typical fuel cell operating condition and should not interfere with the catalytic functions of the electrodes. In addition, they should prevent mixing of fuel and oxidant and seal at least the fuel to the outside atmosphere.
The plates are commonly referred to as bipolar separator plates if both fuel compatible (anode) and oxidant compatible (cathode) functions are combined in the same plate. Plates, which are designed to only provide fuel or only oxidant compatible functions, are commonly referred to as unipolar plates. Such plates can be used to terminate a stack of bipolar separator/membrane electrode assemblies units.
Multiple cell units are most commonly stacked up to increase the overall power in a fuel cell stack. In such an arrangement, fuel and/or oxidant manifolds can be incorporated in the stack. It is often advantageous to compress the fuel cell units together to improve contact of the electrode membrane assemblies to the separator plates, to facilitate fluid sealing and to achieve stable performance.
Fuel cells with solid electrolyte have not reached a significant market penetration, mainly because of high cost, which are due to the expensive materials used for the electrodes, membranes and separator plates and their costly production and assembly. Whereas the cost issues of the electrodes and the membranes are being addressed, the separator plates remain the most expensive component of a fuel cell.
Prior art separator plates use monolithic graphite, which is expensive because of high material costs and the difficulty to form flow field channels in the plates. In addition, these materials are brittle, which makes them prone to cracking and subsequent fuel cell failure. This tendency to crack is aggravated by the need to compress the fuel cell stack and by the temperature changes during operation.
Other prior art separator plates such as disclosed in U.S. Pat. Nos. 5,858,567, 5,863,671 and 5,683,828 describe the use of refractory metals and titanium as materials for the separator plates. These plates are expensive, difficult to form and corrosion products cause problems to the electrodes and membranes.
Further prior art separator plates such as disclosed in U.S. Pat. No. 5,773,161 use stainless steel and nickel plated stainless steel separator plates, which cause unacceptable corrosion when operated at temperatures where liquid water can be present.
U.S. Pat. No. 5,300,370 describes a separator plate formed from flexible graphite foil sheets. These sheets, however, have very poor mechanical properties and are gas permeable.
U.S. Pat. No. 4,214,969 teaches the use of molded aggregates of electrically conductive particles such as carbon and graphite in random polymer matrices. These composites have the drawbacks of relatively high electrical resistance, high surface resistance, low hardness and poor mechanical properties.
It is an object of the present invention to provide improved separator plate structures that overcome one or more of the deficiencies of those of the prior art, particularly those deficiencies discussed above.
Accordingly, there is a strong need for low cost separator plates, which are easy to manufacture. It is, therefore, an objective of the present invention to provide a bipolar separator, which is significantly simpler to fabricate, and does not need machining or etching operations to form the flow field.
It is also an objective of the present invention to lower the cost of the separator plates by extensive use of plastic and by simplified manufacturing.
It is a further objective of the present invention to provide a bipolar plate separator, which has good mechanical properties and a long lifetime.
Advantages of this invention include the low cost of the separator plates due to the use of at least 70% plastic in the separator plates.
Advantages include also the simplicity of manufacturing of the preferred separator plates due to the feasibility to mold the plastic body around the conducting elements and to form the flow field without machining or etching operations.
The above and other objectives are realized in fuel cell separator plates constructed in accordance with the principles of the present invention. The invented separator plates are made of plastic, which is molded around a multitude of conductive elements.
In one embodiment, the plastic part of the plate comprises at least one outlet for the fuel, at least one inlet for fuel, at least one inlet for the air or oxygen, and at least one outlet for the air and/or oxygen.
In accordance with the present invention, there is provided a novel fuel cell for producing electricity by passing hydrogen and oxygen containing fluids over electrodes having an electrolyte body sandwiched between them; and more particularly here is provided a novel separator plate of insulating material and that is positioned adjacent to the electrode of the cell and having recessed surfaces portions forming channels therein for providing flow paths for the respective fluids passing over the electrodes and having a plurality of electrically conducting elements embedded therein, one end of each element being in electrical contact with the adjacent electrode and the other end of the element being in electrical contact with an electrical conductor for deriving electricity produced across the electrodes.
In a preferred embodiment, the conducting elements are made of aligned carbon fiber-polymer composite cylinders, with carbon fibers aligned perpendicular to the main surface of the bipolar plate. In the preferred embodiment, the body of the plate is molded from plastic formed around these conducting elements. In the preferred embodiment, the plastic can be filled or unfilled plastic such as polycarbonate, polyamide, polystyrene, polyphenylene oxide, epoxy, polyphenylene sulfide or the like. In the preferred embodiment, the conducting elements are made of in rod direction aligned carbon fiber/ plastic composites, where the plastic of the composite rods is preferably the same plastic material as used in the body of the plate. Preferred carbon fiber can be high electrically conductive fiber such as pitch based fiber.